1. Field of the Invention
The present invention relates to a sheet conveying device for conveying sheets, and a sheet post-processing apparatus and an image forming apparatus.
2. Description of the Related Art
As image forming apparatuses or sheet post-processing apparatuses have come to perform processes at a higher speed and become more multi-functional, the image forming apparatuses and the sheet post-processing apparatuses have come to be demanded to be able to handle sheets of various thicknesses (thin paper and thick paper) and of various types (special coated paper, different grain directions, and so on). Such an apparatus uses a sheet conveying device as means for conveying (including feeding and discharging) a sheet in the apparatus. The sheet conveying device conveys a sheet by driving and rotating a pair of rollers and passing the sheet between the rollers (nip). In a sheet conveying device, especially upon conveying and discharging soft sheets such as thin paper sheets onto a tray, the sheets may be stacked defectively because the sheets may bend during the discharging process. On the contrary, upon conveying and discharging a hard sheet such as a thick paper sheet onto a tray, if an excessive force is applied by the discharging rollers or near the discharging rollers to the paper to prevent it from bending, traces of the rollers would remain on the sheet, resulting in low quality. Forms and structures of discharging rollers for accommodating different sheets with contradicting properties are already known.
However, the forms and the structures of the conventional discharging rollers require modification of structures depending on user demands, and hence, users sometimes have to go through a replacement process. Some other times, such discharging rollers cannot fully accommodate increasing types of sheets that have recently come to be available, and limits the types of sheets that can be used.
To apply a sheet-strengthening force when a softer sheet is conveyed and to reduce the amount of sheet-strengthening force when a harder sheet is conveyed, Japanese Patent Application Laid-open No. 2010-006538 discloses a plurality of discharging rollers having a discharging ring that is larger in diameter than the outer diameter of the discharging rollers and made of a low-elastic material such as rubber or sponge. In a way, such a structure is similar to the present invention, because both of these inventions take advantage of elastic deformation to change the amount of sheet-strengthening force depending on the type of sheet to be processed. However, Japanese Patent Application Laid-open No. 2010-006538 only discloses changing the amount of the sheet-strengthening force by changes in diameter, and such a structure has not solved the problem of not being able to accommodate a wide range of sheet types. Furthermore, in such a structure, the discharging rollers and the discharging rings have to be created as separate members. Therefore, the apparatus becomes more complex, and the cost is also increased.
The present invention is made in consideration of the above, and an object of the present invention is to provide a sheet conveying device, a sheet post-processing apparatus, and an image forming apparatus that can accommodate a wide range of sheet types including soft sheets such as thin paper and hard sheets such as thick paper.